In the early stage, a scanning apparatus is used to scan the image of a single document. After the image of the document has been scanned, the document should be removed from the scanning apparatus and then a next document could be placed on the scanning apparatus in order to be further scanned. Since the process of manually replacing the document is very troublesome, the conventional scanning apparatus is not feasible to scan a stack of documents. Recently, an automatic document feeder is usually integrated into the scanning apparatus. After a stack of documents to be scanned are placed on the sheet input tray of the automatic document feeder, the automatic document feeder will successively transport the documents to perform a scanning operation without the need of manually replacing the documents. This means of automatically feeding the documents is both time-saving and efficient. The automatic document feeder is also feasible to perform a duplex scanning operation.
Generally, the automatic document feeder has a sheet input tray for placing a stack of documents. The automatic document feeder also has a sheet pick-up module for successively feeding the stack of documents from the sheet input tray to the internal portion of the automatic document feeder in a sheet-feeding direction. For allowing only one document to be fed into the internal portion of the automatic document feeder at each feeding time, the sheet pick-up module has a sheet separation roller and a separation pad or retard roller. The separation pad or retard roller is disposed under the sheet separation roller. The sheet separation roller may provide a friction force to the document that is contacted with the separation pad (or retard roller). The frictional forces between the sheet pick-up module, the separation pad (or retard roller) and the documents should be elaborately controlled. Generally, the friction force between the sheet pick-up module and the document contacted with the sheet pick-up module is greater than the friction force between the documents. In addition, the friction force between the separation pad (or retard roller) and the document contacted with the separation pad (or retard roller) is also greater than the friction force between the documents. As a consequence, only one document is allowed to be fed into the internal portion of the automatic document feeder at each feeding time.
The conventional automatic document feeder, however, still has some drawbacks. For example, when a stack of documents are fed by the automatic document feeder, a double-feeding or multiple-feeding event usually occurs. Since the user fails to realize whether the documents have been completely scanned if the multiple-feeding event occurs, some missing pages are undesirably obtained. Moreover, due to the multiple-feeding event, the documents are possibly damaged, or even the automatic document feeder is damaged.
Moreover, in a case that the automatic document feeder is suffered from the multiple-feeding event, the documents are readily jammed in the internal portion of the automatic document feeder. Due to the friction force between the jammed documents and the separation pad (or retard roller), the jammed documents fail to be pulled out of the automatic document feeder in a sheet-returning direction, which is opposed to the sheet-feeding direction. For releasing the jammed documents, the user needs to open the upper cover of the automatic document feeder to uplift the sheet pick-up module, so that the sheet pick-up module is separated from the upper side of the document. As such, the sheet pick-up module is separated from the upper sides of the jammed documents and the friction force between the jammed documents and the separation pad (or retard roller) is eliminated. Meanwhile, the jammed documents could be effectively released.
Therefore, there is a need of providing an automatic document feeder for detecting the multiple-feeding event and returning the documents in order to obviate the drawbacks encountered from the prior art.